In known material feeders for concrete mixing machines and the like that operate according to the rotor principle, the dry mixture from an open filling hopper arrives at a feed duct by way of a rotor with an upright axis that is sealed off on both sides with rubber discs.
When not rotating, the rotor acts as a closed valve that prevents movement of material from the hopper to an outlet opening or blow-off chamber from which the material is carried away pneumatically. When rotating, the rotor effects transfer of material from the hopper to the blow-off chamber at a predetermined rate. Since the hopper is open to the atmosphere and the material is carried away from the blow-off chamber or outlet by air under substantially high pressure, the rotor also serves as a type of sluice gate or air lock.
Extending through the rotor are cylindrical or segment shaped material chambers that are spaced at like distances from the rotor axis, each open at its top and at its bottom. An inlet that opens downwardly from the hopper, and through which each material chamber can be charged, is located at one point in the orbit of the chambers so that rotation of the rotor brings each chamber, in turn, beneath the inlet opening to have material fall into it from the hopper. The material is encouraged to drop down through the inlet opening by an agitator star in the hopper. As the rotor turns, it carries each filled chamber, in turn, around to an outlet opening that is substantially diametrically opposite the inlet opening. At the outlet opening, pressure air, forced down into the rotor chamber from above, drives the fed material downward out of the rotor chamber and into the blow-off chamber from which the material passes into a feed duct. After about a quarter of a turn, the empty but pressurized rotor chamber is vented shortly before it comes back under the inlet opening, so that material to be fed can fall down into the chamber without being repelled by pressurized air. In a similar manner the process is repeated with each chamber and rotation.
In concrete machines of this type, the water needed for the setting process is added at the end of the feed duct, by means of a spray device.
These known machines have the disadvantage that the rotor and the discs that cover it undergo a high abrasion, since the discs must seal off the rotor outwardly and inwardly from atmospheric pressure and must therefore engage it under high contact pressure. The pressure to be sealed off is equal to the air pressure employed for feeding the dry materials. The large forces that arise from such pressures can only be taken up and carried with a correspondingly heavy structure.
The emptied, pressurized rotor chambers must be vented shortly before they come under the material inlet opening. When wear begins, the pressure air clears a passage for itself along the worn sealing disc, from the blow-out opening to the venting opening, which passage increases with increasing wear; and in the worst case this can lead to destruction of the sealing disc.
Worn out seal plates always lead to dust formation. Dusty air issues at the venting opening and from between the seal plates and the rotor in the region of the pressurized rotor chambers.
By careful analysis it has been established that for heavy structures, the pressure drop .DELTA.p from the feed pressure to atmospheric pressure is above all responsible for abrasion and the appearance of dust. In other words, the prevailing .DELTA.p is the basis for the difficulty in sealing off the rotor and maintaining seals, and for such consequent phenomena as wear and dust.
This is to say that the feed pressure is responsible for dust, wear and the heavy, bulky type of construction heretofore regarded as necessary. Other factors that are also responsible for seal plate erosion and its consequences, such as the fed material, natural dust, and ordinary wear, cannot be influenced by the character of the feed machine.
Machines are also known in which the rotor is surrounded by a jacket that is connected with a filter device by means of connecting hoses. However, in such machines the low pressure around the rotor increases the pressure drop .DELTA.p, and this leads to still worse conditions.
The object of the present invention is to provide a material feeder for feeding dry mixes and the like of the type described above, wherein, to the greatest extent, the deficiencies of prior machines of that type have been eliminated in that the pressure drop .DELTA.p is reduced and in that the distance between the pressurized rotor chamber and the unpressurized chamber can be substantially increased, for example, on the order of half a rotation.
In the materials feeder according to the present invention this problem is solved by the provision of a rotorsluice doser which is characterized by comprising at least two rotors that are known in and of themselves, arranged one over the other, of which at least the upper one has additional chambers acted upon by pressure air.